The present invention relates generally to cellular communication systems. More precisely, the present invention relates to a transmit power supervision method for a base station equipped with a multi-carrier power amplifier (MCPA).
In conventional cellular systems, a base station is allocated a predetermined number of frequency channels for communication with mobile stations. In the base station a separate transmitter is employed for each frequency channel. However, the use of separate transmitters for each frequency channel results in a duplication of parts and an increase in cost due to the additional hardware required. Thereafter, it was realized that the hardware cost per channel could be reduced by using multicarrier transmitters in place of the plurality of single carrier transmitters to transmit a plurality of frequency channels. Since multicarrier transmitters transmit over a broad range of frequencies, they are also sometimes referred to in the art as wideband transmitters. However, for ease of discussion, the transmitters will be referred to herein as multicarrier transmitters.
FIG. 1 illustrates a conventional multicarrier transmitter 100 which may be used to transmit multiple frequency channels from a base station in a radiocommunication system. The conventional multicarrier transmitter 100 operates as follows. A number N of baseband frequency data signals BB1 . . . BBN are modulated by modulators Mod1 . . . ModN, respectively, where the bits associated with each data signal are symbol encoded for transmission, i.e., the modulator generates the corresponding baseband waveform. Each of the modulated data signals is forwarded to a corresponding digital power control module DPC1 . . . DPCN, where each DPC adjusts the signal power level of the corresponding modulated data signal based on the commands provided by the Radio Control Unit 150. More specifically, the power level of each modulated data signal is adjusted such that the absolute power level of each carrier Pk,out at the transmitter is equal to the amount of power required for the carrier to reach a particular mobile station which is to receive the carrier, where k varies from 1 to N and identifies the corresponding baseband frequency data signals BB1 . . . BBN.
The modulated data signals are then forwarded from the digital power control modules DPC1 . . . DPCN to multipliers Mult1 . . . MultN, respectively, where each modulated data signal is upconverted to a corresponding carrier frequency. The upconverted signals are then summed by adder 110. The compound signal produced by adder 110 is then forwarded to the digital-to-analog converter (DAC) 120. The resulting compound analog signal is then passed from DAC 120 through an analog transmitter chain which includes analog amplifier 160, upconverter (not shown), and filters (not shown). Analog amplifier 160 then amplifies the compound signal by a fixed gain Gana. For ease of discussion Gana has been described as the gain of analog amplifier 160, however, one skilled in the art will recognize that Gana represents the total gain of the analog section of the transmitter, including losses due to filters and upconverters. A more detailed discussion of multicarrier transmitters can be found in xe2x80x9cBase-Station Technology Takes Software-Definable Approachxe2x80x9d by Richard M. Lober, Wireless System Designs, Feb. 1998, which is herein incorporated by reference.
Multicarrier transmitters are designed to handle a maximum number of simultaneous carriers N. In designing a multicarrier transmitter, care must be taken to ensure that the instantaneous in-phase power sum, Psum of the N carriers does not exceed the maximum tolerable power of the MCPA. Psum can be calculated using equation (1) below, where Pn represents the power of a specific user, n, in a specific time slot on a specific carrier frequency, and N is the total number of carrier frequencies used by the base station. Normally, Pn is equal to the peak power within the specific time slot.
The instantaneous in-phase power sum of a single time slot for a system having, for example, a constant envelope (such as a GSM system), is given by:                               P          sum                =                              (                                          ∑                                  n                  =                  1                                N                            ⁢                                                P                  n                                                      )                    2                                    (        1        )            
For example, if the instantaneous sum of the N carrier frequencies exceeds the full scale range of the DAC, i.e., the value associated with the greatest digital code that can be converted into an analog value, the DAC will clip the analog signal. Clipping, i.e., preventing the analog signal from exceeding the amplitude corresponding to the full scale range of the DAC will have an adverse effect on the quality of the transmitted signal. However, one skilled in the art will recognize that in practical applications, a system might tolerate a power level which exceeds the DAC""s full scale range by a small amount for short periods of time without suffering a decrease in system performance.
In a multicarrier transmitter with N carrier frequencies, the abovementioned xe2x80x9cclippingxe2x80x9d of the analog signal can be avoided by setting the full scale range of the DAC to 20*log(N) dB above the maximum allowed peak power level of any individual carrier 1 . . . N, since the full scale range set 20*log(N) dB above the maximum power level of any individual carrier represents the greatest power level attainable by the sum of the N carriers.
Designing MCPAs with a high output power is a difficult and expensive task. As the MCPA is designed to have a higher maximum output power, design costs become increasingly more expensive. For a base station operating using time division multiple access (TDMA), the maximum total output power of the base station limits the total output power of the frequency carriers at any time slot. TDMA, as one skilled in the art will appreciate, is a communication technique whereby different signals are assigned to different time slots on the same frequencies. One problem associated with MCPAs designed for a particular output power and operating in a TDMA environment is that a MCPA can only serve a predetermined maximum number of users for the respective particular output power. If more than the predetermined maximum number of users were to be allocated to the MCPA, the MCPA would lose linearity resulting in a decrease in link quality.
FIG. 2 illustrates an exemplary time chart which may be associated with a base station. In FIG. 2, seven frequencies (1-7) in use by an exemplary base station are illustrated over eight time slots. The numbers in the time chart indicate the required output power, in watts, for a mobile unit which is operating at a particular frequency and assigned to a particular time slot. For example, at frequency 1 and time slot 1, the mobile unit requires 4 watts (W). Psum for each time slot is depicted below the time chart.
Assuming, for example, that the maximum power level that can be allocated for each mobile unit is set to 8W, then using Equation (1), the serving MCPA must be designed for at least a maximum output power of 392W. That is, the MCPA must be designed to handle the worst-case scenario of each of the mobile units receiving at their maximum allocated power, 8W. As seen in FIG. 2, the in-phase power sum per time slot, Psum will usually be lower than the maximum output power of 392W. This difference illustrates how the MPCA will not be used efficiently, since it must be designed to handle a worst-case scenario of all users being allocated the maximum output power for a given time slot.
Several techniques have been developed for extending the maximum capacity for which MCPAs have been dimensioned. Load sharing is one such technique. Conventional load sharing is basically a type of load balancing where a user is transferred from one cell which has reached its maximum capacity to another cell which can accommodate the user. This technique avoids overload situations. The following patents illustrate conventional load sharing techniques.
A method of balancing the load among cells which are operating at maximum capacity is described in U.S. Pat. No. 4,670,899, by Brody et al., and entitled xe2x80x9cLoad Balancing for Cellular Radio Telephone System.xe2x80x9d In Brody et al., the loading of various cells is dynamically redistributed by selectively transferring ongoing calls to adjacent cells in accordance with traffic levels in order to reserve channels for handoffs and for new calls. A channel occupancy level for a cell is periodically determined by comparing the number of channels utilized to the number of channels available within the cell. Calls are handed off before all the channels are utilized, thereby allowing at least one or more channels to be reserved for new or incoming calls.
According to the Brody et al. patent, if there is a mobile unit on the periphery of the cell which is also within the range of a neighboring cell, the mobile unit will be transferred to the neighboring cell in order to make room for a new call or an ongoing call associated with a mobile unit which will be handed off to the cell. While Brody et al. provides traffic-based control for call handoffs from one cell to an adjacent cell, handoffs due to load balancing are handled differently from handoffs due to mobile units leaving the cell. This creates a very complex system.
In U.S. Pat. No. 5,625,868 to Jan et al., and entitled xe2x80x9cDynamic Traffic Load Distribution Method,xe2x80x9d the control over a call is transferred from a first satellite to a second satellite having a partially overlapping coverage area with the first satellite when the power consumption level in the first satellite exceeds a certain predetermined level. This is accomplished by switching the channel off in the first satellite and on in the second satellite.
In commonly assigned U.S. Pat. No. 5,241,685 to Bodin et al., and entitled xe2x80x9cLoad Sharing Control for a Mobile Cellular Radio System,xe2x80x9d the entirety of which is incorporated by reference herein, a load sharing method is set forth which is based upon the occupancy of the channels defined by the ratio between the number of occupied channels to the number of available channels.
In commonly assigned U.S. patent application Ser. No. 09/166,159 to Johansson et al., and entitled xe2x80x9cLoad Sharing for MCPA-Equipped Base Stations,xe2x80x9d the entirety of which is incorporated by reference herein, a method is set forth by which users are moved between time slots or between base stations in order to better utilize MCPA output power resources. The power of all transceivers served by a certain MCPA is compared to a threshold value. The threshold value is at least related to the capability of the MCPA in terms of output power. If at no times the sum of the output power exceeds the threshold value, it is assumed that the MCPA can handle all simultaneous transmissions. If, during at least one time slot it is found that the required output power exceeds the capability of the serving MCPA, then a reallocation or load sharing algorithm is invoked. The reallocation algorithm searches for time slots to which a reallocation could be performed within the same base station or number of transceivers served by the MCPA. If no time slots can handle users from the time slot in which the MCPA limit is exceeded, then a load sharing algorithm is activated and one starts to look for transmission resources in transceivers served by other MCPAs.
The present invention distinguishes over the above techniques by providing a more efficient allocation of average output power for a MCPA, the average output power being a weighted average of the actual power over multiple time slots. Periodically, using the more efficient allocation system and method of the present invention, there will be short periods where the desired output power will exceed the maximum tolerable power of the MCPA, PMCPA. These short periods in which the desired output power will exceed PMCPA are handled by an exemplary embodiment of the present invention which produces a reduced margin between the average total power over several time slots and PMCPA. This reduced margin allows the MCPA of the present invention to serve a larger number of users per time slot than conventional MCPAs. Alternatively, the MCPA of the present invention can serve the same number of users as a conventional MCPA, but with higher demands on output power for each user and/or more efficient use of the MCPA""s resources.
According to a first aspect of the present invention, a method for power control in a radio communication system is provided. The method includes determining a power requirement for an output signal; comparing the power requirement with a threshold power value; and modifying the output signal when the power requirement is greater than the threshold power value such that the modified output signal has a power requirement which is less than or equal to the threshold power value.
According to a further aspect of the present invention, a method for power control in a radio communication system is provided. The method includes allocating a respective power for at least one of a plurality of mobile terminals such that an instantaneous power of an output signal for said plurality of mobile terminals will exceed a threshold power value of a power amplifier; determining a power requirement for the output signal; comparing the power requirement for the output signal with the threshold power value; and modifying the output signal when the power requirement is greater than the threshold power value such that the modified output signal has a modified power requirement which is less than or equal to the threshold power value.
According to a further aspect of the invention, a power control system for a radio communication system is provided. The power control system includes a power supervision unit which determines a power requirement for a portion of an output signal and compares the power requirement with a threshold power value; and a signal control unit which modifies the output signal when the power requirement is greater than the threshold power value such that the modified output signal has a modified power requirement which is less than or equal to the threshold power value.
According to a further aspect of the invention, a power control system is provided. The power control system includes means for allocating a respective power level for at least one of a plurality of mobile terminals such that an instantaneous power of an output signal for the plurality of mobile terminals will exceed a threshold power value of a power amplifier; means for determining a power requirement for the output signal; means for comparing the power requirement of the output signal with the threshold power value; and means for modifying the output signal when the power requirement is greater than the threshold power value such that the modified output signal has a modified power requirement which is less than or equal to the threshold power value.
According to a further aspect of the invention, a power control system for a radio communication system is provided. The power control system includes a power supervision unit which determines a power requirement for an output signal and compares the power requirement with a threshold power value. A signal control unit variably modifies the output signal in a digital domain and in an analog domain and a power suppression unit coupled to the signal control unit modifies the power requirement when the power requirement is greater than the threshold power value such that the modified output signal has a modified power requirement which is less than or equal to the threshold power value.
According to a further aspect of the invention, a power control system for a radio communication system is provided. The power control system includes a digital power control module; a digital-to-analog converter; an analog power control module; a radio control unit for adjusting a gain of the digital power control module and for adjusting a gain of the analog power control module; and a power suppression unit coupled to the radio control unit for providing power control commands to the digital power control module.
According to a further aspect of the invention, a method for power control in a radio communication system is provided. The method includes determining a power requirement for an output signal; comparing the power requirement with a threshold power value; variably modifying the output signal in a digital domain and in an analog domain; and modifying the power requirement when the power requirement is greater than the threshold power value such that the output signal has a modified power requirement which is less than or equal to the threshold power value.
According to a further aspect of the invention, a method of power control in a radio communication system is provided. The method includes modulating a plurality of baseband data streams; adjusting a power level of each of the plurality of modulated baseband data streams; upconverting, to a respective carrier frequency, each of the plurality of power modulated baseband data streams to form a plurality of individual carriers; combining the plurality of individual carriers into a single data stream with a power level requirement which is a function of the power levels associated with the plurality of individual carriers; modifying the power level requirement of the single data stream; converting the single data stream into an analog waveform; and variably adjusting the power level of the analog waveform.
It is desirable to limit PMCPA by reducing the margin between the average transmit power over several time slots and PMCPA in order to, for example, reduce the cost of the MCPA. The power control method of the present invention allows for a lower PMCPA, and, therefore, a lower cost MCPA to be employed by a base station. In addition, the present invention does not need to sacrifice quality in order to reduce costs since it is able to maintain sufficient average power.